@isusb can also take just a single letter, i.e., e, as a parameter. Or a full or relative path, or just a file name. As long as the argument resides on a USB device (hard disk or removable stick) @isusb returns 1. @isusb also resolved common types of redirection a few times, three, I think, before it returns its value. This means that if v: is a SUBST for e: and e: is USB then @usb[v] is 1. Same if e: is a USB hard drive (NTFS) and C:\path\junction is a junction linked to e:\path\target then @isusb[C:\path\junction] is 1.